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Introduction to Renewable Energy再生可能エネルギー総論

10Fundamentals of Photochemical Solar Energy ConversionBunsho OHTANI (Institute for Catalysis)

2

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[email protected] Bunsho OHTANI 大谷文章cat Institute for Catalysis 触媒科学研究所hokudai.ac Hokkaido University北海道大学jp Japan 日本

Bunsho OHTANI大谷文章

Fundamentals of Photochemical Solar

Energy Conversion

6

cell

What is cell (denchi)? Interpret in a sense of chemistry.

7

cell

What is cell (denchi)? Interpret in a sense of chemistry.What kind of cells do you imagine?

8

chemistry of fuel cells 8fuel cell (nenryo denchi)

Q What is fuel cell? Interpret the chemistry of fuel cells.

9

chemistry of fuel cells 9fuel cell

fuel cell: producing electricity from fuel

10

chemistry of fuel cells 10mechanism of fuel cell

chemical to electric energy conversion: H2 + 1/2 O2 → H2O

electron

current

inve

rter

H2

AC o

utpu

tair (O2)

anode cathodeelectrolyte

fuel cell

11

chemistry of fuel cells 11merit of fuel cell

QCan we reduce emission of carbon dioxide by using fuel cells?

12

chemistry of fuel cells 12mechanism of fuel cell

chemical to electric energy conversion: H2 + 1/2 O2 → H2O

electron

current

inve

rter

H2

AC o

utpu

tair (O2)


mod

ifier

H2

fuel

gasolinenatural gasmethanolethanol fuel cell

13

chemistry of fuel cells 13fuel for fuel cells

negligible hydrogen (H2) in airH2 is produced in industry, but discarded due to relatively

high cost of storage and transportation.produced via modification with water

CxH4y + 2x H2O → 2(x+y)H2 + xCO2

examples of a modifier for fuel cell cars

14

chemistry of fuel cells 14a fuel cell with a modifier

chemical reaction of modification of fossil fuels and in a fuel cell

Q What is the stoichiometry of combustion of fuel (CxH4y) in engines?

CxH4y + 2x H2O → 2(x+y)H2 + xCO22(x+y)H2 + (x+y)O2 → 2(x+y)H2O

CxH4y + (x+y)O2 → 2y H2O + xCO2

15

chemistry of fuel cells 15

same chemistry/different efficiencyemission of NOx can be avoided by fuel cells

a fuel cell with a modifier


same as combustion in engines



16


higher efficiency if energy can be storedenergy conversion efficiency < 100%spontaneous reaction: use of catalyst

energy conversion

gasoline-engine cargasoline (chemical) → (heat) → (mechanical)

engine engine

fuel-cell car with a modifiergasoline (chemical) → hydrogen (chemical)

modifier

→ (electric) → (mechanical)fuel cell motor

17


spontaneous if Gibbs energy is negative (∆G < 0)

accelerated by CATALYSTwith reduced activation energy (Ea)

Gibbs energy and activation energypo

tent

ial

CxH2yOz

Ea

yH2O + xCO2

E'a∆G

18

chemistry of fuel cells 18catalysts in fuel cell

modifier: catalyst for modificationelectrodes: reaction of hydrogen and oxygen

electron

current

inve

rter

H2

AC o

utpu

tair (O2)


mod

ifier

H2

fuel

gasolinenatural gasmethanolethanol fuel cell

19


same CO2 emission/different efficiencyemission of NOx can be avoided by fuel cells

a fuel cell with a modifier


same as combustion in engines



20

chemistry of fuel cells 20fuel for fuel cells

negligible hydrogen (H2) in airH2 is produced in industry, but discarded due to relatively high cost of

storage and transportation.produced via modification with water

CxH4y + 2x H2O → 2(x+y)H2 + xCO2

production of hydrogen from water by solar light

H2O → H2 + 1/2 O2direct photolysis/photocatalysis/electrolysis with solar cells

21

chemistry of fuel cells 21hydrogen production by solar cells

CIGS (copper-indium-gallium-selenium) photocell

Hondahttp://www.honda.co.jp/news/2003/c031002.html

22

chemistry of fuel cells 22energy conversion


engine engine


modifier


fuel-cell car with hydrogen from solar-cell electrolysissolar light (photo) → (electric) → hydrogen (chemical)

solar cell electrolysis cell

→ (electric) → (mechanical)fuel cell mortorQ Accomplish this scheme.

23



engine engine


modifier




→ (electric) → (mechanical)fuel cell mortor

24



engine engine


modifier




→ (electric) → (mechanical)fuel cell mortor

electric car charged by solar cellsolar light (photo) → (electric) → (mechanical)

solar cell + battery motor

25

chemistry of fuel cells 25electric car with lithium-ion battery

Mitsubishi's electric car "iMiEV"

26

chemistry of fuel cells 26lithium-ion battery

27

chemistry of fuel cells 27silicon solar cell

solar energy is stored by silicon solid-state cell: crystal, amorphous and thin-film silicon are used.

28

chemistry of fuel cells 28dye-sensitized solar cell

Solar energy is stored by wet-type cells.

dye-sensitized solar cells

29

chemistry of fuel cells 29solar energy

hydroelectric power = originally solar energy

30

chemistry of fuel cells 30solar energy

wind electricity = originally solar energy

"windmill"

31

chemistry of fuel cells 31energy conversion in various cells

• primary cell: [chemical] → (electrical)

• secondary cell (rechargeable battery): lead battery/lithium-ion battery(electrical) → [chemical] → (electrical)

• solar cell: silicon solar cell(photo) → (electrical)

• dye-sensitized solar cell(photo) → (chemical) → (electrical)

• fuel cell(chemical) → (electrical)

Fundamentals of

Photochemical Energy Conversion

33

mechanism of microwave cooking

Q Why foods are heated in a microwave oven? Interpret scientifically.

34

microwave heating = rotational excitation

• microwave from magnetron (oscillator): 2.45 GHz (12-cm wavelength)• Water molecules are rotationally excited. Note that water molecules are

rotating even without microwave irradiation and that water molecules in ice are not fixed, i.e., not rotating and thereby no absorption of microwave.

• relaxation: releasing heat as translational energy

energy conversionelectromagnetic wave — rotational energy — translational energy (heat)

keywordrotational excitation and relaxation

35

photoexcitation

Photoreaction proceeds through

excited state.

various excited states• electronic (ultraviolet-visible): photosynthesis,

photocatalysis• vibrational (infrared): water warmed by sunlight• rotational (microwave): microwave oven

the easiest way for making excited states:

photoabsorption

36

What is electromagnetic wave?

Light is electromagnetic wave. Which are electromagnetic waves?alpha beam, beta beam, gamma beam, X ray, ultraviolet light, visible light, infrared light, microwave, radio wave, electron beam

vertical

wavelength

electric field

magnetic field

lightpropa-gation

37

What is electromagnetic wave?

Light is electromagnetic wave. Which are electromagnetic waves?alpha beam, beta beam, gamma beam, X ray, ultraviolet light, visible light, infrared light, microwave, radio wave, electron beam

vertical

wavelength

electric field

magnetic field

lightpropa-gation

38

Q: speed of light

• The shorter the wavelength, the higher the energy.

• The speed of light is constant in vacuum, not depending on its wavelength:

• number of vibration per unit time = frequency (Hz)

• wavelength x frequency = (speed of light)

• (speed of light)/wavelength = frequency

39

Q: speed of light

• The shorter the wavelength, the higher the energy.

• The speed of light is constant in vacuum, not depending on its wavelength:

ca. 3 x 108 m s-1

• number of vibration per unit time = frequency (Hz)

• wavelength x frequency = (speed of light)

• (speed of light)/wavelength = frequency

40

light: wave and particle

• no weight• electron: particle and wave at the same time with weight

(simultaneously) (simultaneously)

diffraction=wave

countable = particle

wave

particle(no weight)

wave

particle(with weight)

41

energy of light

• Even if the total energy is the same, the effect of light may different depending on the energy of each photon.

42

Q: boundary of ultraviolet and visible light

various electromagnetic wave

Q Answer the boundary wavelength of ultraviolet and visible light.

8

7

5

2

-1

-3

-4

-5

-6

-7

-8

-9

-10

-11

-12

-13

-14

6

4

3

1

0

-2

dm

mm

μm

nm

Å

pm

km

m

cm

波長/10 mn

可視光

紫外光

エックス線

赤外光

エネルギー /10 Jn

周波数/10 Hzn

0

1

3

6

9

11

12

13

14

15

16

17

18

19

20

21

22

2

4

5

7

8

10

kHz

MHz

GHz

THz

-33

-32

-30

-27

-24

-22

-21

-20

-19

-18

-17

-16

-15

-14

-13

-12

-11

-31

-29

-28

-26

-25

-23

aJ

fJ

pJ

-9

-8

-6

-3

0

2

3

4

5

6

7

8

9

10

11

12

13

-7

-5

-4

-2

-1

1

μJ

mJ

J

kJ

MJ

GJ

TJ

ガンマ線

マイクロ波

電波

-14

-13

-11

-8

-5

-3

-2

-1

0

1

2

3

4

5

6

7

8

-12

-10

-9

-7

-6

-4

/10 eVn

/10 J moln -1

43

Q: boundary of ultraviolet and visible light

various electromagnetic wave

Q Answer the boundary wavelength of ultraviolet and visible light.

8

7

5

2

-1

-3

-4

-5

-6

-7

-8

-9

-10

-11

-12

-13

-14

6

4

3

1

0

-2

dm

mm

μm

nm

Å

pm

km

m

cm

波長/10 mn

可視光

紫外光

エックス線

赤外光

エネルギー /10 Jn

周波数/10 Hzn

0

1

3

6

9

11

12

13

14

15

16

17

18

19

20

21

22

2

4

5

7

8

10

kHz

MHz

GHz

THz

-33

-32

-30

-27

-24

-22

-21

-20

-19

-18

-17

-16

-15

-14

-13

-12

-11

-31

-29

-28

-26

-25

-23

aJ

fJ

pJ

-9

-8

-6

-3

0

2

3

4

5

6

7

8

9

10

11

12

13

-7

-5

-4

-2

-1

1

μJ

mJ

J

kJ

MJ

GJ

TJ

ガンマ線

マイクロ波

電波

-14

-13

-11

-8

-5

-3

-2

-1

0

1

2

3

4

5

6

7

8

-12

-10

-9

-7

-6

-4

/10 eVn

/10 J moln -1

ca. 400 nm

44

boundary of ultraviolet and visible light

• "Visible" means one can see the light.• The wavelength of light sensible is different individually.• Ordinary speaking, it is approximately 400 nm, certain

people can be sensible for the light of wavelength shorter than 380 nm

45

Q: Why leaves look green?

interaction of light and substancesthe three primary colors: red, green and blueThink complimentary color(s).

46

Q: Why leaves look green?

interaction of light and substancesthe three primary colors: red, green and blueThink complimentary color(s).

(1) Solar radiation contain all light of colors (white).(2) Chlorophyll in leaves absorbs the light of red and blue

(blue-violet).(3) Remaining green light is reflected to make leaves look

green.

Leaves don't absorb green color!

47

photoabsorption spectrum of chlorophyll• electronic absorption spectrum = UV-Vis: extent of photoabsorption is

plotted against wavelength• hint: wavelengths of red and blue lights are 650 nm and 450 nm. • chlorophyll c

48

interaction between light and substance

• electronic/vibrational/rotational energies are "quantized", i.e. discrete level of energy

• energy gap: energy difference between the levels• Only light of energy which is the same as energy gap is absorbed.• photoabsorption = excited state• Excited state must release the energy to go back to ground state.

49

electronic energy

atom as a fundamental particle of substancepositively charged nucleinegatively charged electronselectrostatic interaction between themelectron: wave and particle at the same timequantum theory to interpret bothdiscrete energy level = quantization

50

vibrational energy

molecule: assembly of atomschemical bond: electron(s) existing between nucleus"Spring" is assumed for chemical bonds.quantized

51

rotational and translational energies

molecular rotationMolecules can be rotated if they have dipole moment than changes by rotational motion.Relating to heating in a microwave ovenquantized

translational energymotion of molecules themselvesnot quantized = continuous

52

photoabsorption = excitation

excitation from ground state to excited state

excited state: first, second, third ...

difference in energy = energy gap

excitation = supplying energy to excite

Heat (energy) is too small to excite.

Photons have enough energy for various modes of excitation.

53

fate of excited states

Unstable excited state(s) tends to release energy by(1) deactivation: transition back to the ground state(2) redox reaction(s)

Fundamentals of

Energy ConversionPhotochemicalSolar

55

hydrogen production from water by solar lightlow-cost material may be used as a photocatalyst

H2 and O2 CB

VB

hν

e-

h+

e-

h+

surface

H+H2

H2O

O2

bandgap

photocatalytic decomposition of water

57

Fujishima, A.; Honda, K., Nature 238, 37 (1972).

19,910 citationsat July 3, 2020

58


O2H2

possibly driving

positive ΔG* reactions= energy conversion

*Gibbs energy change

60

principle of photocatalytic reaction

electronic structure of semiconductors and insulatorsconduction and valence bands separated by bandgapphotoexcitation beyond the bandgap

e-e-

h+h+

photo-absorption

recombination

excitation

conduction band

valence band

relaxationreduction

oxidation

relaxation

1) photoexcitation= electron and hole

2) relaxation3a) reduction & oxidation3b) recombination

61

Gibbs energy changeBoth reactions, reduction by e- and oxidation by h+, can be spontaneous, when those reaction proceed independently (separately) and thus,

Photocatalyst particles can drive reaction of ∆G > 0.

photocatalyst

CB

VB

ΔG < 0 absorption

e–

h+

ΔGe < 0

ΔGh < 0

ΔG > 0 absorption

CB

VB

h+

e– ΔGe < 0

ΔGh < 0

ener

gy

62

photocatalysis: two major practical applications

photocatalytic decompositiontarget:

organic compoundssteins (oil)microorganisms

photoinduced (super) hydrophilicitywashable with running wateranti-fogging

ΔG < 0

63

mixed metal oxidesK4Nb6O17 (Domen et al.)Na2Ti6O13 (Inoue et al.)

1990

examples with mixed metal oxides

1998 NaTaO3 (quantum efficiency 50%/Kudo et al.)

1969 TiO2 photoelectrode (Fujishima and Honda)TiO2 particles (Sato)SrTiO3 particles (Domen et al.)

1980

2001 two-step visible-light photolysis with iodidemediator (Abe et al.)

2004 one-step visible-light photolysis by GaN:ZnOsolid solution (Domen et al.)

ultravioletlight

history of photocatalytic water splitting

64

Domen's group (The University of Tokyo)

hydrogen evolution by GaN:ZnO photocatalyst

65


1990




1980



ultravioletlight


66


no information on →electrolytes

↓

67

Checked by several electrochemists:

63) A. Nozik, Nature, 257, 383 (1975). 64) M. S. Wrighton, D. S. Ginley, P. T. Wolczanski, A. B. Ellis, D. L. Morse, and A.

Linz, Proc. Natl. Acad. Sci. U.S.A., 72, 1518 (1975).

following papers to reproduce the results

68

TiO2 Pt

↑O2 ↑Ｈ2

e-

UV

TiO2

↑O2

e-

↑Ｈ2

UV

Pt

high pH low pH

hidden message of Honda-Fujishima paper

requirement of BIAS potential, i.e., electric FIELD in the BULKto separate photoexcited electron and positive hole (= charge separation) to drive the reactions of positive Gibbs energy change

J. Photochem. Photobiol. C: Photochem. Rev., 11, 157 (2010)

69

photosynthesis

Solar energy is stored as chemicals.

70

Z-sheme of natural photosynthesis

spatial separation of redox reactions

71

Gibbs energy changeBoth reactions, reduction by e- and oxidation by h+, can be spontaneous, when those reaction proceed independently (separately) and thus,

Photocatalyst particles can drive reaction of ∆G > 0if CHARGES are separated.

photocatalyst

CB

VB

ΔG < 0 absorption

e–

h+

ΔGe < 0

ΔGh < 0

ΔG > 0 absorption

CB

VB

h+

e– ΔGe < 0

ΔGh < 0

ener

gy

72

density of states

absorption edge: corresponding to band gapapparently the edge is not SHARP due to distribution of "density of states"

DOSdensity of states

negligible DOS at the edges = less photoabsorption

73

photoabsorption of formaldehyde (HCHO)

calculation results using molecular orbital theorynegligible special overlapping of orbitals between n and π*

= little HOMO-LUMO photoexcitation (change orbital)

O 2py

O 2s

O 2pz

O 2pxC 2pz

sp2

HOMO

LUMO

C 2py

C 2px

C 2s

orbi

tal e

nerg

y (a

tom

ic u

nit)

74

charge separation in a particle

• photoabsorption = electron-positive hole creationat the SPATIALLY SAME position= exchange (switch) of orbital to higher-energy one

• coulombic force upon separation= necessity of an internal electric field

e-

h-

75


1990




1980



ultravioletlight


76

two-step (Z-scheme) photocatalysis

77

Domen's group (The University of Tokyo)

hydrogen evolution by GaN:ZnO photocatalyst

78

chemistry of fuel cells 78energy conversion in various cells

• primary cell: [chemical] → (electrical)

• secondary cell (rechargeable battery): lead battery/lithium-ion battery(electrical) → [chemical] → (electrical)


• dye-sensitized solar cell(photo) → (chemical) → (electrical)

• fuel cell(chemical) → (electrical)

79

chemistry of fuel cells 79pseudo photochemical solar energy conversion


• secondary cell (rechargeable battery):lithium-ion battery

(electrical) → [chemical] → (electrical)

80

"lecture" web page

https://pcat.cat.hokudai.ac.jp/cgi-bin/lecture/showLectures_e.cgior search by "hokudai ICAT pcat", then "講義"

81

format

Please send email in Japanese or English within 72 hours

to: [email protected]: re20200717-XXXXXXXX

[email protected](full name)(nickname)(comments on today's lecture)(question if any)[blank line](answer for question 1)(answer for question 2)(answer for question 3)

2020/07/17 16:30-18:00"lecture" web pageformat�Bunsho OHTANI�大谷文章�スライド番号 5cellcellfuel cell (nenryo denchi)fuel cellmechanism of fuel cellmerit of fuel cellmechanism of fuel cellfuel for fuel cellsa fuel cell with a modifiera fuel cell with a modifierenergy conversionGibbs energy and activation energycatalysts in fuel cella fuel cell with a modifierfuel for fuel cellshydrogen production by solar cellsenergy conversionenergy conversionenergy conversionelectric car with lithium-ion batterylithium-ion batterysilicon solar celldye-sensitized solar cellsolar energysolar energyenergy conversion in various cellsスライド番号 32mechanism of microwave cookingmicrowave heating = rotational excitationphotoexcitationWhat is electromagnetic wave?What is electromagnetic wave?Q: speed of lightQ: speed of lightlight: wave and particleenergy of lightQ: boundary of ultraviolet and visible lightQ: boundary of ultraviolet and visible lightboundary of ultraviolet and visible lightQ: Why leaves look green?Q: Why leaves look green?photoabsorption spectrum of chlorophyllinteraction between light and substanceelectronic energyvibrational energyrotational and translational energiesphotoabsorption = excitationfate of excited statesスライド番号 54photocatalytic decomposition of waterスライド番号 56Fujishima, A.; Honda, K., Nature 238, 37 (1972).Fujishima, A.; Honda, K., Nature 238, 37 (1972).スライド番号 59principle of photocatalytic reactionGibbs energy changephotocatalysis: two major practical applicationshistory of photocatalytic water splittinghydrogen evolution by GaN:ZnO photocatalysthistory of photocatalytic water splittingFujishima, A.; Honda, K., Nature 238, 37 (1972).following papers to reproduce the resultshidden message of Honda-Fujishima paperphotosynthesisZ-sheme of natural photosynthesisGibbs energy changedensity of statesphotoabsorption of formaldehyde (HCHO)charge separation in a particlehistory of photocatalytic water splittingtwo-step (Z-scheme) photocatalysishydrogen evolution by GaN:ZnO photocatalystenergy conversion in various cellspseudo photochemical solar energy conversion"lecture" web pageformat

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